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Lecture 4.3_Biogeography and Global Provincialism in Mammalian Distributions

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hello again Professor Jared rathel here

and this is lecture 4.3 in ABS 470 it's entitled

entitled

biogeography and Global provincialism

and mammalian distributions

so when we think about mammalian

distributions around planet Earth why is

it that we see mer mccophagus meaning

ant and Termite eating mammals on nearly

every continent on the planet

yet they're all different

how do scientists explain the abundance

of marsupials in Australia and South

America but their absence or scarcity on

Northern continents

why are there camels in Asia North

Africa and South America

what factors led to the present

distribution of primates from Japan to

Africa to South America so these are

biogeographical questions this lecture

is going to align with chapter 5 which

is entitled biogeography please note and

this is also in canvas but you may skip

the sections in your textbook entitled

biogeographic inference as well as the

section simply called examples

biogeography is the study of the

distribution of organisms both living

and extinct around our planet the most

basic unit of biogeography is the

species range that is to say the

complete area over the planet over which

individuals of a particular species

occur so as an example this is Puma

concalor common names include the

mountain lion the cougar the panther the

Catamount all the same species so Puma

concalor is distributed from British Columbia

Columbia

all the way down through the southern

tip of Argentina

as you likely recognize uh species

ranges are Dynamic they're not fixed in

time they're they're changing especially

as humans rapidly transform both the

surface of the planet as well as its

atmosphere our climate an endemic

species is one that's restricted in its

range only to a circumscribed area it's

only found there so for example we can

say that lemurs are endemic to

Madagascar they're only found in

Madagascar lemurs are not found across

the channel in Mozambique

hindemism is really important in

conservation biology because it helps us

identify biological hot spots regions

around the world that have really unique

flora and fauna that are worth

conserving like Madagascar a second

pattern of interest is known as disjunct

distribution so that simply means that

there's a gap in the range of related

species or some taxonomic claim so as

you already know

marsupials are abundant on the continent

of Australia and the island of Tasmania

as well as in South America and then we

have the Virginia opossum here in North

America you may also recall that the

oldest metatherian fossil yet recovered

is synodelphies which dates at 125

million years ago and was recovered in

Asia of all places

further the oldest confirmed marsupial

fossil recovered dates at 65 million

years ago and that's in North America so

it suggests that the ancestors of modern

marsupials dispersed from Asia to North

America and then down into South America

and we know that South America

represents the Cradle of marsupial evolution

evolution

marsupials then make their way across a

once green Antarctica and it may have

been just one colonizing species that

makes it to Australia some 50 million

years ago and the winner of that

sweepstakes their descendants are then

going to go on and fill almost every

conceivable mammalian ecological niche

in Australia

ecological biogeography it's going to

focus on the current distributions of

species but it's going to seek to

explain those distributions in terms of

community level interactions between the

species and their environment so one

common line of inquiry within ecological

biogeography is thinking about species

richness so that refers to how many

species are in the system in the region

and it leads to questions like why is it

that we see so many mammalian species

distributed between about 23 degrees

north of the equator down to 23 degrees

south in this Tropics in the tropical

band here so think about out the Amazon

rainforest or East Africa so you can see

species richness for mammals is quite

High uh in these these regions uh

whereas places like Antarctica they have

very few mammals

okay further when we think about

ecological biogeography we can think

about islands and why certain Islands

have relatively High species richness

compared to other Islands so and we're

going to come back to Island

biogeography but it has to do with the

size of the island as well as its

distance from the mainland

so answers to ecological biogeographical

questions are going to involve evolutionary

evolutionary

adaptations so ecological biogeography

is going to frequently entail studying

patterns of morphological

physiological and or life history

variation among mammals in different

places if we tabulate the number of

species and major clades of virtually

any group of animals or plants for that matter

matter

they occur in different Continental

regions of the earth and in doing so two

patterns are going to emerge first

different regions Harbor distinct

taxonomic assemblages so there's there's

endemism on a worldwide scale so think

kangaroos they're endemic to Australia

Gibbons in Southeast Asia sloths in

South and Central America

second pattern there are dramatic

differences in species richness amongst

the Continental regions some regions

represent centers of diversity While

others simply do not so these

observations together with knowledge

about phylogenetic or evolutionary

relationships demonstrate this

provincialism these provinces of Life on

planet Earth and it's a pattern that's

evident in the tetrapod fossil record

since the early Mesozoic some 250

million years ago it was way back in

1876 that the famous naturalist who

worked out both the mechanics and the

profound implications of natural select

action independent of Darwin Alfred

Russell Wallace

he's the first to divide the world up

into faunal regions for which he

identifies six regions each with a

distinct assemblage of species those are

the pale Arctic the knee Arctic the

neo-tropical what he deems as the

Ethiopian Oriental and Australian

so Wallace noted that although the

distance between the islands of Bali and

lombok in Indonesia is only a little

over 20 miles in fact on a clear day you

can see from Bali to lombok from one

Island to the next

the faunal assemblages on those two

islands are Worlds Apart so

Bali and everything north of what has

now been deemed Wallace's line

everything up here is Asians part of the

Oriental species assemblage so think uh

crab eating macaques meanwhile

everything's south of Wallace's line is

Australian so think about tree kangaroos

on Papua New Guinea all right you know

the drill please put me on pause and

check out Wallace Darwin's forgotten

Frenemy which is embedded in canvas

thanks next I'm going to take you on a

really quick tour of Wallace's sixth

faunal regions so for each of the

regions I'll explain where it's at on

the planet the major biomes that

comprise each region and then I'll

highlight any endemic families in the

region that is to say families of

mammals that are found in that region

and no place else

so we'll begin with the pale Arctic this

is the largest of the faunal regions and

it consists of the old world that is to

say Europe Russia and Northern China

it's separated from the Ethiopian region

uh by the deserts of the Middle East

it's separated from the Oriental region

by the Himalayan mountains which are

pretty formidable barrier and then it's

separated from the knee Arctic region by

the bearing straight

so there's an Eastward band of tiger uh

coniferous forest that's going to run

about 60 degrees north latitude

let's see the southwestern pale Arctic

includes the Mongolian steps

while there's temperate deciduous forest

and Chaparral forests in Europe and then

the pale Arctic is home to just one

endemic sub-family the blind mole rats

shown top left

the pale Arctic mammals represent a

mixture of meat Arctic

Ethiopian as well as Oriental elements

so think

servids bovids

ersids and felids but there are

certainly unique lineages uh within

these families so uh the bactrim camel

shown here or the Siberian tiger uh

bottom middle or the adorable giant

panda the knee Arctic extends from

Beyond the Arctic Circle here in

Northern Canada all the way down to the

central Mexican plateau and it's going

to include the island of Greenland the

knee Arctic is separated from the pale

Arctic as mentioned on the previous

Slide by the bearing straight

and the knee Arctic is separated from

the neotropics uh by this Central

American transition zone uh this

tropical uh rainforest like the pale

Arctic the knee Arctic consists of

tundra up north and tiger coniferous

forest there's also deciduous forests on

our Eastern Seaboard as well as

grasslands and then uh Chaparral forests

and deserts where we live in the Southwest

Southwest

many of the families that we find in the

knee Arctic also exist in the pale

Arctic because these two regions have

often been connected uh by the Bering

land bridge uh which multiple times uh

in the past have connected uh Siberia to

Alaska there are relatively few

mammalian families that are endemic to

the knee Arctic but there are two and

those are the families of the pronghorn antelope

antelope

as well as the mountain Beaver the Neo

Tropics or the nootropics extends from

Central Mexico all the way down to the

tip of South America the region is

mostly isolated it's surrounded by

oceans uh but it's Northern boundary

roughly coincides with a transition uh

from Zurich or dry subtropical desert to

moisture tropical forests okay so that's

a tough boundary uh to cross

tropical wet Forest of the Amazon

dominate the neotropics with grasslands

and deserts to the south of the Amazon

and then Alpine habitats that are

associated with those High impressive

Andes mountains along the western margin

of South America the neotropics are a

true Center of diversity they have large

numbers of mammal families and many of

them are endemic so think uh guinea pigs

the cavids this is a wild guinea pig

here uh sloths the two and three-toed

sloths that we've covered uh New World

monkeys we're going to cover primates

next we spent quite a bit of time

thinking about the opossums the many

different opossums that are found in

South America and then lastly the

selenodons are endemic to the region and

they're found in the Caribbean moving

over to the continent of Africa uh

Wallace deemed this final region the

Ethiopian region and it's going to refer

to sub-Saharan Africa that is to say the

portion of the African continent which

is south of the expansive Saharan Desert

including the island of Madagascar so

the Saharan Desert and the deserts of

the Middle East that's going to form the

transition zone the boundary between the

Ethiopian faunal region and that of the

pale Arctic the biomes in Africa there

is a massive swath of tropical

rainforest here uh in the Congo uh

southwestern Africa is characterized by

deserts so the Kalahari Desert excuse me

uh we've got Savannah in Eastern Africa

and then over in Madagascar there's rain

forest on the east side and Savannah on

the west and those habitats are going to

run parallel along that Island

so there is a great diversity of mammals

in Africa and it's been attributed in

part to restriction of plyo pleistocene

Extinction so just we didn't see the

extinctions uh in the plyocene and

pleistocene in Africa that we saw in

other regions like North America

further there's been a relatively recent

diversification of moderately large

bodied animals in Africa so indomism in

Africa and the Ethiopian region is

really high on the island of Madagascar

which you probably guessed uh endemic

species to Madagascar include the Lemurs

which we'll talk about next when we talk

about primates as well as these really

cool bats uh they are called uh sucker

footed bats and if you look closely at

their hands their little suction cups to

adhere to the surfaces of leaves so

really cool endemic bats in Madagascar

as well as the 10 Rex are endemic uh to

this region on the mainland uh Ethiopian

endemics include the gold in moles the

elephant shrews you remember we went

over those as well as the aardvark and

then there's an incredible uh radiation

of terrestrial Set artiodactyls uh

hoofed animals uh like uh the wildebeest

which we'll cover later in the semester

the Oriental region is comprised of the

Indian subcontinent

southeast Asia and the Malay archipelago

north and west of Wallace's line

so the Oriental region the primary biome

is tropical rainforest and it is a

stunningly beautiful Forest as somebody

who spent quite a bit of time in Thailand

Thailand

the mammal diversity in the orient is

quite High there are five endemic

families including the dermoptera the

kalugos and the scandentia the tree

shrews both of which we've just covered

as well as the aptly named hognose bats

the Gibbons and the Tarsiers uh both

primates that will cover next

and then the last point that I'd like to

make is the Oriental region is really

across roads and so it's going to

consist of Ethiopian families

um so the Asiatic lion lions evolved in

Africa uh the Asiatic lion was once

incredibly widespread uh sadly there's

only a few hundred uh Asiatic Lions left

their marooned uh in the gear Forest of

India and then uh the ursids this is a

pale Arctic family the bear family an

oriental species is the sun bear which

was also once widely distributed uh but

now the sun bears main stronghold is in

Cambodia the Australian final region

obviously includes the continent of

Australia as well as New Guinea and

Tasmania so it's bounded by AI Wallace's

line here in the Northwest which we've

discussed and then it's surrounded by

oceans which means it's highly isolated

there is a variety of biomes in

Australia it's mostly desert but we have

tropical rainforest in northeastern

Australia and places like Cairns and

Townsend as well as in New Guinea there

are temperate deciduous forests in the

south east there's Chaparral forests in

the South there's temperate rainforests

in eastern Tasmania so as you likely

recognize because Australia is so

isolated uh it's going to share very few

of its mammalian families with other

areas so endemism is quite High

endemics include the monotreams like the

spiny echidnas and the duck build

platypus as well as many marsupial

orders including uh the dazzy

euromorphia like this tiger Coral uh the

notoricta morphia like the marsupial

mole here the uh Bilby or the

rabbit-eared a Bandicoot uh the die

Proto dantia which are the Kangaroos and

the wallabies

so the only recent route of exchange for

Australian mammals is by Crossing

Wallace's line and there's two euthyrian

groups that have done just that the bats

and the murid rodents who we will talk

about so they've invaded a good example

is this species here this is the Gould's

mouse named after Stephen J Gould it was

thought to be extinct for 150 years but

was recently found thriving on numerous

small islands off the western coast of

Australia so that's your feel-good story

for the lecture and our final faunal

region is actually not one that was

identified by Wallace but it's pretty

important with respect to mammalian

distributions and that is the oceanic

faunal region it's comprised of mammals

that live on isolated did remote Islands

far from Continental land masses so

think about the Hawaiian hoary bat or

these really beautiful rats this is the Slender

Slender

slender-tailed Cloud rat from the island

of luson in the Philippines

so it's going to include insular species

as well as our aquatic species those

living close to coasts like the manatees

and the dugongs as well as pelagic

species fully marine species that live

out in the open ocean

so Oceanic Islands particularly those

that are volcanic in their origin they

tend to have very few mammal species

only those mammals that could reach

there uh by wind wave or Wing

um so in Hawaii for example there's only

two endemic mammals the Hawaiian hoary

bat as well as the monk seal those

islands that do have mammals they're

mostly bats and small rodents or mammals

that have

been dispersed to the region By Us by

humans it's their dispersal has been

facilitated by humans so think about our

menageries so pigs goats uh the Norway

ship rat that's Road everywhere with us

around the planet

among the Marine groups we know the

serenians the manatees and the dugongs

they occur along tropical coasts uh the

pinnipeds like these sea lions they're

gonna breed on packed Ice uh in the

Arctic and uh Antarctic regions as well

as near shore rocks or coastal areas and

then we have the cetaceans uh like this

beautiful breaching humpback whale and

uh these spinner dolphins which are

amazing to see

these are pelagic species meaning

they're denizens of the open ocean the

suggestion that the continents of planet

Earth drift over vast periods of time

was actually first proposed way back in

1915 mean by Wagner but it wasn't

accepted until the 1960s because up

until that time there was no geological

mechanism known that could account for

such movement plate tectonics is going

to provide that mechanism so the Earth's

crust including the continents and the

ocean floor is made up of Rocky plates

that are actually floating on denser

partially melted molten mantle Rock

there are some 10 of these major plates

and then there are numerous smaller

plates they're separated from one

another uh by ridges trenches and faults

so as heat from Earth's core

radiates outward it creates convection

currents right uh in this viscous mantle

Rock which is going to move these land

masses so here's a really nice

visualization created by the California

Academy of Sciences that demonstrates

the breakup of the supercontinent of

Pangea and into uh godwana land and then

the movement of our continents to their

current positions so this abiotic

process happened over the past 200

million years really important with

respect to mammalian Evolution because

it's going to create isolation and then

adaptive radiations and it's going to

really be the a driver for this

provincialism these centers of diversity

that we've been discussing that really

shape mammalian distributions today so

please take the two minutes uh and check

out this visualization

the climate or the long-term weather

patterns of a particular area on planet

Earth is the result of interactions

between sunlight the atmosphere the land

masses themselves as well as ocean

currents so a portion of the sun's

infrared radiation that reaches the

Earth is reflected back off the surface

and then it's trapped by carbon dioxide

and other greenhouse gases like methane

this blanket of gases in the atmosphere

is what creates the Greenhouse Effect

and it makes our planet livable inhabitable

inhabitable

but as I'm sure you're aware the amount

of CO2 in our atmosphere is rapidly

Rising so we're currently at 420 parts

per million that's the most CO2 in our

atmosphere in the past 400

000 years as measured by Antarctic Ice

cores so 400 000 years that's longer

than we've even been homo sapiens

although the Earth is warming at an

unprecedented rate today at several

times in Earth's history levels of

greenhouse gases fell and continents

occupied positions that blocked the flow

of warm equatorial ocean water to the poles

poles

and we got ice ages so for example in

the Carboniferous much of Southern

godwana was covered by glaciers which

had dramatic impacts on the pelicosaurs

and I hope you remember them way back

from module one

more recently

from the mid Maya scene on which is

about 15 million years ago the world

became cooler and drier until roughly

two million years ago when it was

plunged into the pleistocene Ice Age so

glaciation during this period was most

dramatic in the knee Arctic in North

America which you can see here where

there were these giant ice sheets these

incredible glaciers that covered most of

modern Canada and the northern United

States and it shifted the tundra and the

Taiga the coniferous forests Southward

between about 1.7 million years ago up

until as recently as 10 000 years ago

continental glaciers Advanced and then

retreated at least 4 four times giving

rise to a cycle of glacial and then

interglacial periods that culminated in

the recent distributions of many modern

mammals that are still responding to the

last glacial Retreat and now of course

the big question is are they going to be

able to respond to anthropogenic climate

change the term dispersal has two

closely related meanings in ecology the

first is when individuals or small

groups of individuals leave their natal

area the place where they're born and

raised to go and breed elsewhere so

think about young male wolves that

disperse from their natal pack to form

their own new pack so these types of

dispersals they're going to occur within

the lifetime of an individual organism

and we refer to this as ecological dispersal

dispersal

dispersal in the sense that we're

interested in today in the biogeographic

sense refers to a species

expanding its range so species dispersal

species dispersal can be passive

dispersal so think rodents rafting to an

island and then they hit the sweepstakes

and they land on this new island with no competition

competition

or species dispersal can be active so

that's when individuals of a species

actively disperse via a corridor route a

corridor route is one that provides

minimal resistance to the passage of

animals between two areas whereas a

filter route is going to do just that

it's going to filter out some animals

it's only going to allow some animals to

pass through the filter route so for

example flying bats are really good at

getting through filters the filter of

Wallace's line as well as animals that

have really large home ranges so think

about Puma con color

from British Columbia all the way down

to South America

um so a great example of a filter route

is the Panamanian land bridge that forms

between North and South America about

3.5 million years ago

and facilitated the Great American

biotic interchange so this Ted Ed is

going to do a very nice job of breaking

that down for you so check it out in the

pliocene about three and a half million

years ago this land connection between

North and South America was

re-established by the emergence of the

Isthmus of Panama right here which is

going to initiate

extensive dispersal of mammals between

the two continents so it's important to

note Panama was initially Savannah very

similar to Savannah habitats in the

North and the South so initially it's a

corridor for some mouth were dispersing

mammals such as Horses and deer as well

as Savannah dwelling mammals from South

America like the glyptodons and the

ground sloths however Panama is going to

develop into a tropical rainforest

during the pleistocene and then it's

going to be less of a corridor and more

of a filter which is only going to allow

some animals through

so from the north

rabbits and squirrels rodents the

canaans the Bears the raccoons they were

able to cross this filter and make their

way into South America whereas

whereas

shrews and beavers and pronghorns did

not make it

okay thinking uh uh the other way

um porcupines and opossums were able to

invade uh North America along with the

nine banded armadillo however most

armadillos and anteaters and the

tropical sloths uh were not able to make

it through that filter so the difference

between a corridor and a filter route

really nicely exhibited uh By the Great

American biotic interchange which was

initially a corridor and then became

more of a filter just as Extinction is

going to reduce the species richness of

a clade evolutionary diversification is

going to increase species richness so

diversification in this sense is nothing

more than speciation but the most

noteworthy cases of speciation those in

which it happens very rapidly and we see

this incredible explosion of new

descendant species in a geographically

restricted area we're going to refer to

these as adaptive radiations

so some spectacular examples of adaptive

radiations when

um you know one species makes it to a

new area and we see this incredible

flowering of new species include the

Lemurs on the island of Madagascar of

which there are over a hundred species

there's my favorite the II the new world

monkeys another uh wondrous adaptive

radiation which we'll cover in our next

lecture and then of course the

Australian marsupials which we've talked

about at length of which there are over

240 species in these cases the ancestors

of these clades they disperse into this

new region and they encounter little

competition in this new range or they

get there and they out-compete the

residents uh that are in that Niche a

striking regularity in mammalian

phylogeny is the number of times that

ecologically similar species have Arisen

in different areas and from different

ancestors because of

convergent evolution so this is a topic

that I've discussed multiple times this

semester thus far for example

mircophagy which I discussed on the

first slide mammals that specialize on

eating ants and termites with its

specialized cranial morphine apology and

those elongated sticky tongues that's

evolved in six different mammalian

orders including the numbats pictured

here which are of the marsupial order

dazzy uromorphia uh the anteaters

there's a giant ant eater here which is

in the order pelosa the pangolins which

are the folidota the aardwolves and the

Order of Carnivora the aardvarks uh the

tubule Eden Tata and the echidnas which

are monotreams

so your semester Capstone project which

I will describe in detail later it's

going to allow you to explore convergent

evolution in much greater depth it's a

fascinating topic since the 19th century

a number of regularities have been noted

in the ways that mammals vary with

geography many of these patterns have

been codified as Eco Geographic

rules but as your textbook notes none of

the patterns that I'm about to describe

are invariant there's a lot of

exceptions and some of these rules

really have questionable generality and

all of these rules are going to to be

the result of complex historical and

environmental factors

thus we call them rules but they're only

rules really in the loosest sense so

we'll start with the island Rule and

it's formulated on the observation that

small mammals on Islands tend to evolve

larger body sizes than their close

relatives their ancestors on the mainland

mainland

we see this insular or Island

gigantism on the island of Flores uh in

Indonesia with its giant rat so here's

the Flores giant rat

um considerably larger than rats that

are found on the Southeastern Asian mainland

mainland

conversely the island rule states that

large mammals are going to show the

opposite Trend so Island species usually

evolve smaller body sizes then their Mainland

Mainland

counterparts so the eco-evolutionary

underpinning goes like this

for large mammals like elephants

resource limitations on small Islands

small land masses should be the most

intense selection pressure and therefore

it's going to favor reduced body size

there just isn't as much forage for

elephants on Islands so there's going to

be selection for smaller and smaller elephants

elephants

however smaller mammals when they arrive

on Islands they're going to have reduced

inter-specific competition I.E

ecological release which is going to

favor the evolution of larger body sizes

for these small mammals which are less

likely to experience resource

limitations uh because of smaller areas

that Islands represent so if we go back

to the island of Flores we see insular dwarfism

dwarfism

in pygmy elephants which are now extinct

and then what I find Most Fascinating

is uh insular dwarfism in

homophorensis The Hobbit which likely of

involved from much larger bodied Homo

erectus and this little hominid it may

have died out as recently as 50 000

years ago so it may have overlapped uh

with uh the First Peoples uh in

Indonesia this is such an amazing

discovery that I want you to take a few

minutes and just check this out it is

just so cool

one of the earliest

eco-geographic rules was that of Bergman

way back in 1847 and he observed that

body sizes of both mammals and birds

tend to increase with increasing

latitude mammals are going to get larger

as we move farther north and south away

from the equator

the argument underlying this latitudinal

size gradient is based on the superior

heat conserving capacity of larger

bodied endotherms so a typical large

mammal has a much lower surface area to

volume ratio they have more volume

relative to their surface than a small

mammal hence smaller surface area across

which to lose body heat at cold temperatures

temperatures

so this subspecies of red fox here this

is the Arabian Fox it has a much lower

body mass and a lot more surface area

relative to its volume which is going to

allow it to dissipate heat

um compared to

um this arctic fox which has is

considerably chunkier and has a larger

body mass and is better able to conserve

heat less surface more mass in those

Arctic temperatures

so uh Blackburn and Hawkins uh from 2004

this is figure

5.18 in your textbook uh they found uh

that average annual temperature is the

strongest predictor among six factors

they evaluated

um in predicting average log body mass

uh in North American mammals okay so

here are these really cold regions and

you can see the mammals tend to be much

larger bodied and as average temperature

warms uh mammalian body sizes get

smaller so this is a negative uh

polynomial model extending bergman's

reasoning about

thermoregulatory adaptations in

endotherms in 18 177

177

Alan uh proposes this rule that mammals

as well as Birds again living in cold

climates have shorter appendages than do

their close relatives living in warmer

environments so long Limbs and Tails and

certainly ears they're going to increase

the surface area for heat dissipation in

mammals which is adaptive as a cooling

mechanism in hot dry environments like

the Sonoran Desert certainly appears to

be the case in uh jackrabbits in

comparison to Arctic hairs in

1883 a gloeger noticed an apparent

correlation between the plumage color of

closely related Birds and the humidity

levels of their habitats with birds with

darker feathers more frequently found in

human environments and birds with

lighter feathers more likely to exist in

dry areas

so in mammals the rule would apply to

their pillage their fur color so for

example white light colored polar bears

inhabiting the very dry Arctic in

contrast with the dark brown grizzly

bear and the humid or at least

relatively human boreal forests of North

America and Eurasia so gloeger's rule

has been documented in primates uh but

other than that it's generality it

appears quite Limited

Rapaport in 1982

1982

noted that the

latitudinal breadth of a species range

tends to increase as we move from

equatorial species up in Latitude

towards the poles so for example

consider the Y distribution of the

Caribou range of her tyrandis right

really widely distributed species

obviously at high latitudes uh around

the Arctic Circle in comparison to the

very restricted range of the elds deer

pictured here which lives in tropical

southeast Asia much closer to the Equator

Equator

so it's argued uh that species living at

high altitudes like the Caribou they're

adapted to this relatively broad range

of seasonal environmental conditions

right so uh you know in June we've got

the Sun never setting and then um you

know obviously in December uh perennial

Darkness so these huge swings uh that

Caribou have to deal with in contrast

tropical species are adapted to a more

stable climate thus they may have

evolved towards more ecological

specialization and have narrower

distributions I'll conclude this lecture

with the latitudinal gradient in species

diversity so this is probably the first

Global ecological path pattern that was

described by naturalists so species

diversity I.E the number of species per

unit area is going to decrease as we

move away from the equator up towards

the poles

so for example there are less than 40

species of mammals that inhabit

latitudes above the Arctic Circle but as

we move towards the equator

mammalian species richness increases

and as we get all the way here to Costa

Rica we're gonna Peak at about 160

species of mammals that inhabit uh these

lush tropical Costa Rican rainforests so

there's quite a few hypotheses uh that

have been put forth to explain this uh

you know pretty

um solid observation that exists not

only in mammals but in a whole variety

of different taxa these hypotheses

aren't mutually exclusive and I just

wanted to go over two really quickly the

first is the out of Tropics model and it

posits that most mammalian clades

evolved originated in the tropics in the

tropics they experience low Extinction

rates so species tend to persist for

longer periods of time and from the

tropics they're going to move out and

colonize subtropical and temperate

regions as a function of distance

and then another argument is the

metabolic hypothesis and it's going to

argue that the higher primary

productivity of tropical habitats just

results in higher rates of speciation as

well as lower Extinction rates because

they're more stable in comparison to

temperate habitats when you think about

these habitats just during the

pleistocene with uh you know glaciers uh

advancing and then retreating we see a

lot of variability over time and with

that I will bring lecture 4.3 to a close

I am super excited about our next

lecture so I'll be discussing mammalian

mating systems and parental investment

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